1. Field of the Invention
The present invention relates to an exposure apparatus that transfers a pattern of an original to a substrate by exposure, and particularly, to a scanning exposure apparatus that performs exposure while scanning an original and a substrate. An exposure apparatus of this type is used for manufacturing semiconductor devices, liquid-crystal devices, MEMS elements, image pickup devices, and magnetic heads.
2. Description of the Related Art
The following is a description of related art concerning the manufacture of semiconductor devices, which is considered to be the field where device miniaturization is most advanced.
In a manufacturing process of semiconductor devices, a front-end process and a back-end process to be described below are repeated to form transistors. A front-end process involves applying a resist layer over various thin films formed on a silicon substrate. A back-end process includes a lithography step for transferring a circuit pattern onto the resist layer by exposure, a resist development step, and an etching step. Particularly, the exposure apparatus used in the lithography step significantly contributes to the miniaturization of transistors for enhancing the degree of integration. Up to this day, semiconductor devices have been becoming more and more miniaturized without showing any sign of stopping, and the degree of integration has been doubling every two years in accordance with the so-called Moore's Law. Until we face unavoidable technical issues, further miniaturization is expected to continue.
To achieve miniaturization in the lithography step is substantially equivalent to increasing the resolution of a lens in the exposure apparatus. According to Rayleigh's formula, resolution can be expressed as k1·λ/NA. In this case, λ represents a wavelength of a light source, NA represents a numerical aperture of a projection lens, and k1 represents a coefficient determined as a result of processing. In order to utilize the NA to a maximum extent, exposure apparatuses have been shifted from a square field angle full plate exposure method (i.e. stepper) to a scanning exposure method (i.e. scanner). A scanning exposure method applies a strip-shaped exposure area that utilizes the lens to its maximum fabricatable diameter so as to increase the width of the field angle. In a scanning exposure method, an exposure process is performed while scanning is performed in the vertical direction. In addition to the advantage of achieving a larger field angle by utilizing the maximum diameter of the lens with the strip-shaped exposure area, a scanning exposure method is advantageous in view of achieving miniaturization, such as having the capability to continuously maintain a focused state during a scanning exposure process and to obtain a large field angle in the scanning direction.
On the other hand, in addition to the improvement of resolution, another significant improvement in exposure apparatuses is productivity. In particular, an effective way to improve the productivity in a scanning exposure apparatus is to raise the maximum shifting rate of a wafer-holding stage in order to increase the speed of the scanning exposure process. With respect to the shifting rate of the wafer stage, a reticle stage that holds a reticle serving as an original is shifted at a rate that is in inverse ratio to the projection magnification of the lens. Under the present circumstances, it is most common that the projection magnification is ¼×, the shifting rate of the wafer stage is substantially 0.5 m/s, and the shifting rate of the reticle stage is 2 m/s. For the improvement of productivity, a further increase in the scanning rate is expected.
In a scanning exposure apparatus, exposure light transmitted through the reticle must be accurately projected onto a chip of the wafer through the projection lens. The reticle stage and the wafer stage have individual position measuring systems that use laser interferometers, and are driven in accordance with predetermined scan profiles. During a scanning exposure process, vibration occurring from the shifting of the stages, load deformation in stage guiding faces, and uneven driving forces, for example, can cause positional errors. Errors that have occurred on the respective stages are evaluated with deviation indicators with respect to target positions on the stages. Furthermore, an indicator called synchronization accuracy that indicates the difference between the deviation of the reticle and the deviation of the wafer is used in order to evaluate an alignment error in the relative position of the stages that should originally be aligned in synchronization with each other.
An error in synchronization accuracy is a kind of displacement unique to scanning exposure apparatuses. The low frequency component of the spatial frequency in the error can cause scan distortion (i.e. distortion within the field angle), and the high frequency component can cause contrast deterioration (i.e. deterioration in the image quality). Moreover, the direct-current component becomes an amount of displacement in an entire shot and directly affects the alignment accuracy. Japanese Patent Laid-Open No. 2003-273007 proposes a control method in which distortion data generated in each shot under a predetermined scanning rate is set by a measuring unit for every shot, and a correction drive value is calculated on the basis of the distortion data, so that correction is performed on a shot-by-shot basis. This correction is advantageous in that displacement and distortion occurring in a scanning exposure process can be corrected, and is particularly effective for reducing distortion errors that are highly repeatable.
However, the correction method disclosed in Japanese Patent Laid-Open No. 2003-273007 has several problems. First of all, in order to calculate a correction value to perform feedback on the correction using the exposure result, it is necessary to actually print a pattern on the wafer and measure the printed pattern with the measuring unit.
Furthermore, because the correction value is set on the basis of the exposure result, an accurate correction amount can only be determined within a measurable exposure range, that is, the area corresponding to the shot size. Performing a scan alignment on the reticle stage and the wafer stage based on TTL method using an alignment mark located outside the exposure range is problematic in that the correction is not possible since the alignment mark is outside the correction range. Even though Japanese Patent Laid-Open No. 2003-273007 proposes a unit that extrapolates a correction value for outside the exposure range using an approximate function, such a unit is intended for improving the follow-up of the correction value in the exposure range and does not have a sufficient function for actively and accurately performing positional correction for outside the exposure range.
According to the knowledge of the present inventor, there has been arisen a new problem, which was not an issue in the past. Specifically, the scanning rate of scanning exposure apparatuses has been increasing generation by generation for the purpose of improving productivity as mentioned above. In addition, the demand for higher overlay accuracy of patterns has been becoming higher year after year. Due to these two technical trends, the effect of a propagation delay of light is becoming a problem, which was not an issue in the past. While exposure light passes through the reticle and reaches the wafer through a lens, the wafer stage is shifted by an amount equivalent to the time that takes for the exposure light to travel from the reticle to the wafer. Therefore, even if the reticle stage and the wafer stage are in an idealistic control state where control deviations of the stages are both zero, a displacement dependent on the shifting rates still occurs. This displacement becomes a definitive error that is uniquely determined by the light velocity and the stage shifting rates.
In the related art, displacement errors occurring due to various factors such as a deformation error, vibration, and uneven force included in the exposure result are collectively measured, calculated, and corrected. If there is a large error component in the components of the correction value, a small error component will unfavorably be hidden. If a large error component can be determined definitively, it is desirable that the large error component be removed as a definitive error so that a small error component can be subsequently corrected with high precision. Since a propagation delay of light taken into consideration in the present invention corresponds to a definitive error, a propagation delay of light should be removed in advance separately from other error factors in order to improve the accuracy of the correction.
Here, an amount of delay in the light velocity, which is a definitive displacement error, will be estimated.
In a current scanning exposure apparatus, a propagation time tl of light between the object and the image can be expressed as follows:tl=L·(na(1−γ)+γng)/c  Expression 1where L indicates an object-image distance between the reticle serving as an original and the wafer serving as a substrate subject to exposure, which is approximately 1 m, γ indicates a glass containing rate of a reduction projection optical lens disposed between the reticle and the wafer, which is about 90% with respect to the center of the optical axis, c indicates a light velocity that is 2.99×108 m/s, na indicates a refractive index of air that is 1.00, and ng indicates a refractive index of glass that is 1.47. Consequently, tl=4.7×10−9 sec.
An amount of displacement d of the wafer stage is expressed as follows:d(v)=v·tl  Expression 2where v indicates the scanning rate of the exposure apparatus, which is 1 m/s at the wafer stage. Consequently, d=4.7 nm.
Since the scanning direction is inverted between adjacent shots, the amount of displacement in Expression 2 doubles between adjacent shots so as to become 9.4 nm.
In recent years where an alignment accuracy of 10 nm or less is required in exposure apparatuses, an amount of displacement caused by a delay of light is not negligible. The necessity to increase the scanning rate for further throughput improvement will lead to a larger amount of displacement.
FIG. 4 is a conceptual diagram of a pattern that has been transferred onto a wafer W1 by exposure. To improve the productivity, a scanning exposure apparatus reciprocally scans the reticle stage serving as an original so that the exposure process can be successively performed at a high operating rate. Thus, the exposed shots form an array in which shots SA and shots SB respectively transferred and exposed as a result of positive (+) direction scanning and negative (−) direction scanning of the reticle stage are alternately arranged. In FIG. 4, the arrows represent the scanning directions of exposure light with respect to the corresponding shots (the shifting directions of the stage being opposite to the scanning directions). If there is no propagation delay of light and ideal synchronization control of the reticle stage serving as an original and the wafer stage is achieved, there should be no displacement among adjacent shots. However, in a case where an exposure process is performed at a scanning rate that is affected by the aforementioned propagation delay of light, the horizontally adjacent shots are displaced from one another in the scanning direction, resulting in a defective arrangement as shown in FIG. 4.
In an exposure apparatus of the related art, the propagation delay of light is not taken into account for the calculation of the synchronization accuracy. For this reason, even if an error in the synchronization accuracy is ideally reduced to zero, a displacement still appears in the exposure result. In other words, in the calculation method of the related art, the synchronization accuracy and the exposure result vary from each other.